One type of multilayer coating known for giving substrates such properties consists of a metallic functional layer having reflection properties in the infrared and/or in solar radiation, especially a metallic functional layer based on silver or on a metal alloy containing silver.
In this type of multilayer coating, the functional layer is thus placed between two dielectric films each comprising in general several layers that are each made of a dielectric material of the nitride type, and especially silicon nitride or aluminum nitride, or of the oxide type. From the optical standpoint, the purpose of these films that flank the metallic functional layer is to “antireflect” this metallic functional layer.
However, a blocker film is sometimes interposed between one or each dielectric film and the metallic functional layer, the blocker film placed beneath the functional layer in the direction of the substrate protects said functional layer during an optional high-temperature heat treatment of the bending and/or tempering type, and the blocker film placed on the functional layer on the opposite side from the substrate protects this layer from any degradation during the deposition of the upper dielectric film and during an optional high-temperature heat treatment of the bending and/or tempering type.
As a reminder, the solar factor SF of glazing is the ratio of the total solar energy entering a room through this glazing to the total incident solar energy, and the selectivity S corresponds to the ratio of the light transmission TLvis in the visible of the glazing to the solar factor of the glazing, and is such that: S=TLvis/SF.
Currently, there are low-E thin-film multilayer coatings comprising a single functional layer (denoted hereinafter by the expression “multilayer coating comprising a single functional layer”) based on silver, having a normal emissivity ∈N of around 2 to 3%, a light transmission in the visible TL of around 65% and a selectivity of the order of 1.3 to 1.35 for a solar factor of about 50% when mounted in a conventional double-glazing unit, such as for example on face 3 of the following configuration: 4-16(Ar-90%)-4, consisting of two 4 mm glass sheets separated by a gas-filled cavity containing 90% argon and 10% air with a thickness of 16 mm, one of the sheets of which is coated with the multilayer coating comprising a single functional layer, namely the sheet furthest to the inside of the building when considering the incident direction of the solar light entering the building, on its face turned toward the gas-filled cavity.
A person skilled in the art knows that by positioning the thin-film multilayer coating on face 2 of the double glazing (on the sheet furthest to the outside of the building when considering the incident direction of the solar light entering the building and on its face turned toward the gas-filled cavity) he can reduce the solar factor and thus increase the selectivity.
Within the context of the above example, it is then possible to obtain a selectivity of around 1.5 with the same multilayer coating comprising a single functional layer.
However, this solution is unsatisfactory for some applications since the light reflection in the visible, and in particular the light reflection in the visible seen from outside the building, is of a relatively high level, above 20% and about 23 to 25%.
To reduce this light reflection, while still maintaining energy reflection, or even increasing energy reflection, a person skilled in the art knows that it is possible to introduce one or more layers, which is/are absorbent in the visible, into the multilayer coating and more particularly into one or more of the dielectric films.
It appears that certain rules have to be respected when positioning a multilayer coating in a multiple glazing unit, according to the position of the layer or layers comprising a single functional layer absorbent in the visible—it is this that forms the subject matter of the present invention.
It should be noted that the prior art already teaches the use of such layers which are absorbent in the visible in multilayer coatings comprising several functional layers, in particular international patent application WO 02/48065 which relates to the usage of such layers which are absorbent in the visible in a multilayer coating resistant to a heat treatment of the bending/tempering type.
However, because of the complexity of the multilayer coating and the amount of material deposited, these multilayer coatings comprising several functional layers are more costly to manufacture than multilayer coatings comprising a single functional layer.
Moreover, also because of the complexity of this multilayer coating comprising two functional layers, the teaching of the above document cannot be directly transposed for designing a multilayer coating comprising a single functional layer.